Re: [PATCH 07/55] media: imx8-isi: Stop abusing of min_buffers_needed field

[Tomasz Figa]

On Tue, Nov 28, 2023 at 7:26 PM Benjamin Gaignard
<benjamin.gaignard@xxxxxxxxxxxxx> wrote:
>
>
> Le 28/11/2023 à 10:35, Tomasz Figa a écrit :
> > On Tue, Nov 28, 2023 at 6:31 PM Benjamin Gaignard
> > <benjamin.gaignard@xxxxxxxxxxxxx> wrote:
> >>
> >> Le 27/11/2023 à 18:07, Laurent Pinchart a écrit :
> >>> Hi Benjamin,
> >>>
> >>> Thank you for the patch.
> >>>
> >>> On Mon, Nov 27, 2023 at 05:54:06PM +0100, Benjamin Gaignard wrote:
> >>>> 'min_buffers_needed' is suppose to be used to indicate the number
> >>>> of buffers needed by DMA engine to start streaming.
> >>>> imx8-isi driver doesn't use DMA engine and just want to specify
> >>> What do you mean, "doesn't use DMA engine" ? The ISI surely has DMA
> >>> engines :-)
> >> I have done assumption on drivers given if they use or dma_* functions.
> > I suspect the use of vb2_dma_sg_plane_desc() and
> > vb2_dma_contig_plane_dma_addr() may be more correlated to whether
> > there is a DMA involved or not. Usually V4L2 drivers don't really have
> > to deal with the DMA API explicitly, because the vb2 framework handles
> > most of the work.
>
> Unfortunately isn't not true either, for example verisilicon driver use
> these function but doesn't need DMA engine.

That sounds weird. Why would a driver that doesn't have a DMA engine
need to obtain a scatterlist or the DMA address of the buffer?

> I haven't found yet a 100% criteria to decide if driver use or not DMA engine
> so I plan to fix case by case given maintainers remarks.

Yeah, there probably wouldn't be a way that would give one a 100%
certainty, although I'd still insist that the two functions I
mentioned should be close to that. Of course a driver can use those
functions for some queues, while other queues would be pure software
queues, e.g. for some metadata - a simple grep is not enough. Is that
perhaps the case for the verisilicon driver?

Best regards,
Tomasz

>
> Regards,
> Benjamin
>
> >
> > Best regards,
> > Tomasz
> >
> >> I have considers that all PCI drivers are using DMA engine and
> >> I don't know the design for each drivers so I hope to get this information
> >> from maintainers and fix that in v2.
> >> If imx8-isi driver needs a minimum number of buffers before start streaming
> >> I will do a v2 and use min_dma_buffers_needed instead.
> >>
> >> Regards,
> >> Benjamin
> >>
> >>>> the minimum number of buffers to allocate when calling VIDIOC_REQBUFS.
> >>>> That 'min_reqbufs_allocation' field purpose so use it.
> >>>>
> >>>> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@xxxxxxxxxxxxx>
> >>>> CC: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
> >>>> CC: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>
> >>>> CC: Shawn Guo <shawnguo@xxxxxxxxxx>
> >>>> CC: Sascha Hauer <s.hauer@xxxxxxxxxxxxxx>
> >>>> CC: Pengutronix Kernel Team <kernel@xxxxxxxxxxxxxx>
> >>>> CC: Fabio Estevam <festevam@xxxxxxxxx>
> >>>> CC: NXP Linux Team <linux-imx@xxxxxxx>
> >>>> ---
> >>>>    drivers/media/platform/nxp/imx8-isi/imx8-isi-video.c | 2 +-
> >>>>    1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/drivers/media/platform/nxp/imx8-isi/imx8-isi-video.c b/drivers/media/platform/nxp/imx8-isi/imx8-isi-video.c
> >>>> index 49bca2b01cc6..81673ff9084b 100644
> >>>> --- a/drivers/media/platform/nxp/imx8-isi/imx8-isi-video.c
> >>>> +++ b/drivers/media/platform/nxp/imx8-isi/imx8-isi-video.c
> >>>> @@ -1453,7 +1453,7 @@ int mxc_isi_video_register(struct mxc_isi_pipe *pipe,
> >>>>       q->mem_ops = &vb2_dma_contig_memops;
> >>>>       q->buf_struct_size = sizeof(struct mxc_isi_buffer);
> >>>>       q->timestamp_flags = V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC;
> >>>> -    q->min_buffers_needed = 2;
> >>>> +    q->min_reqbufs_allocation = 2;
> >>>>       q->lock = &video->lock;
> >>>>       q->dev = pipe->isi->dev;
> >>>>